Process and Apparatus for the Liquefaction of Carbon Dioxide

ABSTRACT

Apparatus for carbon dioxide liquefaction comprising a flow channel for carbon dioxide passage from an inlet port to an outlet port. The channel comprises a plurality of compressors ( 2, 5, 8 ) and coolers ( 4, 7, 9, 10, 13 ) arranged in series with an expansion chamber ( 14, 15 ) in said flow channel downstream of the final compressor ( 8 ) and cooler ( 9, 10, 13 ). The apparatus also comprises a recirculation channel ( 16 ) arranged to return gaseous carbon dioxide from said expansion chamber ( 15 ) into said flow channel ( 3 ) upstream of said final compressor ( 8 ) and cooler ( 9, 10, 13 ).

The present invention relates to a process for the production of liquidcarbon dioxide and apparatus for use in said process.

Carbon dioxide (CO₂) is a gas produced as a by-product in largequantities in certain industrial operations, e.g. the manufacture ofammonia, or power generation by coal or gas power plants. Release ofthis by-product into the atmosphere is undesirable environmentally as itis a greenhouse gas. Much effort has thus been made towards thedevelopment of techniques for the disposal of CO₂ in a way other thansimple release to the atmosphere. One technique of particular interestis to pump the CO₂ into porous sub-surface strata (i.e. rock), e.g. downan injector well in an oil field.

Subsurface disposal can be simply into porous strata or beneficialadvantage of the subsurface disposal can be realised if the stratum intowhich it is disposed is hydrocarbon-bearing as the injected CO₂ servesto drive hydrocarbon (e.g. oil or gas) in the stratum towards theproducer wells (i.e. wells from which hydrocarbon is extracted).Injection of CO₂ is thus one standard technique in late stage reservoirmanagement for achieving enhanced recovery of hydrocarbons.

The quantities of carbon dioxide involved when disposal is by subsurfaceinjection are immense, generally of the order of millions of tonnes.This poses problems in terms of transporting the CO₂ from the site atwhich it is created to the site at which it is injected, especiallywhere the injection site is offshore. Carbon dioxide at ambienttemperatures and pressures is gaseous and, if transported batchwise,such voluminous containers are required that the process would beunfeasible. While transport by pipeline might in some circumstances befeasible, the required infrastructure is expensive. It is thereforedesirable to transport the carbon dioxide, especially to offshoreinjection sites, batchwise in liquid form.

Transport of liquid carbon dioxide is however not a problem- orexpense-free exercise. If the liquid CO₂ is not refrigerated, thepressures required to maintain it in the liquid state are high (60-80bar A) making the required wall thicknesses of the pressurizedcontainers high and making such containers for large scaleunrefrigerated liquid CO₂ transportation immensely expensive. Transportof liquid CO₂ at sub-ambient temperatures reduces the required pressuresand required container wall thicknesses but is expensive sincerefrigeration is required and, as carbon dioxide has a solid phase,there is a risk that solid carbon dioxide can form. Solid carbon dioxideformation makes CO₂ transfer by pumping problematic and, due to therisks of pipe or valve blockage, potentially dangerous.

Thus in balancing the economies of refrigeration and container cost andavoiding the risk of solid CO₂ formation, in any given circumstancesthere will generally be a temperature and pressure which is optimal forthe liquid CO₂ in the containers, e.g. a temperature which is belowambient and a pressure which is above ambient but still sub-critical(the critical point of CO₂ is 73.8 bar A). Typically for large scaleliquid CO₂ transport the optimum temperature is likely to be in therange −55 to −45° C. and the pressure is likely to be 5.5 to 7.5 bar A,i.e. corresponding to the position in the phase diagram for CO₂ which isjust above the triple point in terms of temperature and pressure. Thetriple point for CO₂ is 5.2 bar A and −56.6° C. Lower temperatures andpressures raise the risk of dry ice formation; higher pressures requiremore expensive containers; and lower pressures raise the risk of gas orsolid formation.

While small scale production (e.g. currently typically 0.1 tonne/year)of liquid carbon dioxide is relatively trivial, generally involving two,three or four cycles of compression and cooling/expansion, bulkproduction at the level of millions of tonnes is by no means trivialsince, starting with a gas which is, or is majoratively, carbon dioxideat or near ambient temperature and pressure, transforming this startingmaterial to liquid carbon dioxide at the temperatures and pressures thatare desirable for bulk transport involves significant pressurization andenergy removal.

We have now found that production of liquid carbon dioxide in bulk andat temperatures and pressures desirable for bulk transport may beeffected in an environmentally friendly and efficient manner byproducing liquid or dense fluid (i.e. super critical) carbon dioxide attemperatures and pressures above the desired values, expanding it togenerate liquid carbon dioxide at the desired values and cold gaseouscarbon dioxide which is recycled into the compression andcooling/expansion cycles bringing down the mean enthalpy of the CO₂ flowthrough those cycles. In this way no expensive coolant is required andCO₂ release into the atmosphere may be avoided.

Thus viewed from one aspect the invention provides a process for theproduction from a feed gas which comprises carbon dioxide of liquidcarbon dioxide at a desired temperature and pressure which temperatureis below ambient, above the triple point temperature for carbon dioxideand below the critical point temperature for carbon dioxide and whichpressure is above ambient, above the triple point pressure for carbondioxide and below the critical point pressure for carbon dioxide, saidprocess comprising: feeding said feed gas into the entry port of aliquefaction apparatus having a flow path from said entry port to anexit port connected to an expansion chamber; flowing said gas as a fluidalong the flow path through said apparatus and subjecting said fluid toa plurality of compression and cooling cycles whereby to generate liquidor super-critical carbon dioxide having a temperature and pressure abovesaid desired temperature and pressure; passing said liquid orsuper-critical carbon dioxide through said exit port into said expansionchamber whereby to generate in said chamber gaseous carbon dioxide andliquid carbon dioxide at said desired temperature and pressure; andrecycling said gaseous carbon dioxide into fluid flowing through a saidcompression and cooling cycle; and optionally withdrawing said liquidcarbon dioxide at said desired temperature and pressure from saidexpansion chamber.

One or more of the compression and cooling cycles, preferably all suchcycles, may additionally involve an expansion step which will of coursefurther cool the fluid. It is especially preferred that the fluidflowing to each compression step is monophasic, i.e. gaseous or densefluid (super-critical); however it is optional whether the product ofthe final compression and cooling step comprises liquid carbon dioxideor dense fluid carbon dioxide.

If desired the expansion chamber may be detachable from the liquefactionapparatus and may thus serve as the transport vessel for the liquidcarbon dioxide. Preferably however the expansion chamber has a liquidremoval port through which the liquid carbon dioxide may be withdrawninto a transport vessel. The expansion chamber may be any componentsuitable for expansion, such as an expansion valve and the like.

The gaseous carbon dioxide which is recycled is preferably passedthrough one or more heat exchangers to draw energy from the fluid flowbefore being returned into the fluid flow at an upstream point.

Since the feed gas may contain impurities, e.g. water, nitrogen, etc.,it is desirable that the fluid flow be subjected to one or moretreatments to remove these. Depending on apparatus design, these removalsteps may cause some consequential removal of carbon dioxide from theapparatus other than as liquid CO₂. Careful design however can result inonly minimal such non-liquid carbon dioxide removal.

In general, at least two (e.g. 2 to 8, preferably 4) compression stepswill be required to transform the fluid into liquid or super-criticalcarbon dioxide. It is preferred to effect water removal after at leastone compression step and before the final compression step, e.g. betweenthe second and third compression steps, typically after the cooling stepfollowing the prior compression step. It is especially preferred toeffect water removal before each compressor step. Desirably the CO₂ gasis dried to ppm level by adsorption after the last separator.

Water should be removed to avoid hydrates, freezing of water, corrosionand droplets of water in the compressor feed. The solubility of water inCO₂ gas decreases with higher pressure and lower temperatures. Water canbe removed in several ways, e.g. using separators or by passage througha water absorbent or adsorbent bed or filter. Preferably most of thewater is removed in separators, after each compression and cooling step.

For water removal by condensation and separators, the CO₂ gas withliquid contaminants (e.g. water and also other liquids such as liquefiedheavy hydrocarbons) enters a separator where the condensed liquids aredrawn off from the base of the separator and the CO₂ leaves the top ofthe separator in gaseous form.

Desirably the dried gas leaving a separator or separators is led throughan adsorption unit before passing to the next compression step. In orderto permit continuous operation, it is desirable to have two or more suchadsorption units arranged in parallel so that one may be regenerated(for example by passing hot gas through it) while another is in use. Thegas used for regeneration will typically be gaseous carbon dioxide whichis being recycled. The hot, moist carbon dioxide leaving the unit beingregenerated may desirably be recycled into the fluid at an upstreampoint, e.g. between the first and second compression steps, preferablybetween a compression step and the subsequent cooling steps.

Especially preferably the last free water is removed in a separatorbefore the last compressor step at a pressure between 20 and 40 bar andat a temperature close to the hydrate formation curve, that is, between10° C. and 15° C. Desirably the CO₂ gas is dried to ppm level byadsorption after the last separator.

Where the feed gas contains further gases that, at ambient temperature,undergo a phase change to liquid phase at a temperature lower than thatof carbon dioxide, e.g. gases such as nitrogen, oxygen, methane orethane, these gases are desirably removed prior to the last expansion.

For such feed gases it is therefore desirable that the liquefactionprocess include a step in which such “volatiles” are removed. Thispreferably occurs following a compression or cooling step whichgenerates liquid CO₂, or more preferably a fluid, which consists of asmuch gas as is to be removed in the removal step and the rest in theliquid phase. If heat is rejected at pressures higher than the CP in thesupercritical phase, the removal of volatiles will be done after thefirst expansion step, where the fluid is in the two phase region underthe CP with a low gas fraction.

The removal of volatile components may be done in a separation columnafter heat rejection close to the dew point line. At transport pressuresof 6-7 bar A only small fractions of volatiles, typically 0.2-0.5 mole %can be included in the product to ensure that dry ice is not formed. Ifmore volatiles are present in the feed they should be removed. Aseparator tank could be used; however, a separator column is preferablyused to avoid venting of large quantities of CO₂ to the atmosphere. Thecooling in the condenser is provided by vaporisation of liquid CO₂ atintermediate pressure stages or from the product tank. As a rule ofthumb the loss of CO₂ will be equal to the amount of volatiles in thefeed.

To further enhance volatile removal, some or all of the liquid CO₂withdrawn from the separator column may be warmed (e.g. in a reboiler)and returned into this separator column. The reboiler may alternativelybe integrated in the separator column.

The cooling units arranged to cool the fluid flow may use recycledcarbon dioxide as the cooling fluid. However the cooling units in atleast the first compression and cooling steps conveniently use anexternally sourced fluid, typically water, e.g. sea, river, or lakewater or ambient air.

The apparatus used in the process of the invention preferably comprisesgas tight conduits joining the various operating units, i.e.compressors, coolers, heaters, heat exchangers, etc. and provided withappropriate valves. Ideally the flow path has only one entrance port(for the feed gas) and only one exit port (for the liquid CO₂); howeverexit ports for water or volatiles removal will be present in certainembodiments.

The feed gas for the process of the invention is preferably majoritivelycarbon dioxide (on a molar basis), e.g. 55 to 100% mole CO₂ or 70 to 95%mole CO₂, especially at least 70% mole CO₂, more especially at least 90%mole CO₂, particularly up to 95% mole CO₂. More preferably the feed gascontains less than 0.5 mole % of volatile components and less than 0.1mole % of water. Preferably the water content is not in excess of 50 ppmby weight. As mentioned earlier, the carbon dioxide produced as aby-product in ammonia production or the carbon dioxide captured fromcoal or gas power plants is particularly suitable.

Viewed from a further aspect the invention also provides apparatus forcarbon dioxide liquefaction comprising a flow channel for carbon dioxidepassage from an inlet port to an outlet port, said channel comprising aplurality of compressors and coolers arranged in series, with anexpansion chamber in said flow channel downstream of the finalcompressor and cooler and with a recirculation channel arranged toreturn gaseous carbon dioxide from said expansion chamber into said flowchannel upstream of said final compressor and cooler.

The apparatus of the invention is conveniently provided with the furtherstructural components discussed above in connection with the process ofthe invention.

Embodiments of the invention will now be discussed further by way ofillustration and with reference to the following non-limiting Examplesand the accompanying drawings, in which:

FIG. 1 shows a schematic of one embodiment of the apparatus of theinvention; and

FIG. 2 shows a schematic of a preferred embodiment of the apparatus ofthe invention.

FIG. 1 is a schematic of the main elements of the apparatus. Feed gascontaining 100 mole % carbon dioxide is supplied from a source (notshown) to the input port of conduit 1. The gas is fed to a firstcompressor 2 and then to a first intermediate cooler 4 via conduit 3.Second stage compression and cooling is performed by second stagecompressor 5 and cooler 7 (connected by conduit 6) and the final stageof compression is achieved using compressor 8 and cooler 9. Heat isextracted in each of the coolers 4, 7 and 9 using ambient air or water(conduits not shown) as the cooling medium.

The fluid output from the last compression stage is communicated to afirst input 10 a of heat exchanger 10. The first output 10 b of heatexchanger 10 is connected to first input 13 a of a second heat exchanger13. In addition, the first output 10 b is connected via conduit 12 andexpansion valve 11 to the second input 10 c of heat exchanger 10. Theexpansion valve 11 is arranged to expand and cool the first output 10 bfrom heat exchanger 10. This acts to cool the fluid flowing between 10and 10 b. The recycled carbon dioxide gas flowing between the thirdinput 10 e and 10 f will also cool the fluid flowing 10 a to 10 b. Thesecond output 10 d is connected to conduit 6 between compressor 5 andcooler 7 whereby to recycle the gas drawn off down conduit 12.

The first output 10 b from heat exchanger 10 passes through a furtherheat exchanger 13 and to expansion valve 14. The fluid is then expandedto the transport pressure by expansion valve 14 and fed into theseparator 15. The gas phase (or flash gas) is returned via conduit 16and heat exchangers 13 and 10 respectively to the conduit 3 arrangedbetween the first compressor 2 and first cooler 4. The arrangement ofthe two heat exchangers 10 and 13 acts to cool the flow of fluid passingbetween 10 a, 10 b, 13 a and 13 b because the flash gas in conduit 16and the expanded supply gas in conduit 12 will be at a lowertemperature. This increases the efficiency of the process.

The liquid phase separated in separator 15 is output via output 17 to astorage or transport vessel (not shown).

Expansion of pressurised fluids as mentioned above may convenientlyinvolve use of a Joule-Thompson valve. Alternatively, an expansionturbine may be used for expansion of the pressurised fluids as mentionedabove. This will increase the energy efficiency of the process.

Referring to FIG. 2, feed gas is delivered into the inlet port ofconduit 18 in the apparatus and thence into separator 20 which serves tocondense water which is removed through conduit 21. The gas then passes,via conduit 22, to the first stage compressor 23 and to first stageintermediate cooler 24. This first stage of water removal, compressionand intermediate cooling is repeated as shown in FIG. 2 by separator 25,second compressor 26 and second cooler 27. The output of the secondintermediate cooler 27 is passed through a heat exchanger 28 via conduit29 where the temperature of the feed gas is further reduced by heatexchange with gaseous carbon dioxide recycled from downstream in theapparatus.

Intermediate coolers 24 and 27 reject heat to sea water.

The feed gas flows from heat exchanger 28 to separator 30 via conduit31. Water removed in separators 25 and 30 is returned to the firstseparator 20 via conduits 32 and 33.

Water is removed from the feed gas by means of the three separators 20,25 and 30 by condensation. It is highly desirable to remove water fromthe feed gas to avoid hydrate formation and corrosion which can occur ifsignificantly more than 50 ppm (wt.) water is present. Removal of wateralso increases the efficiency of the process.

Feed gas is then fed from the third separator 30 via conduit 34 to oneof two water adsorption units 35 a and 35 b where the water content isreduced still further to approximately 50 ppm.

At any one stage, one water adsorption unit is in use while the other isbeing regenerated (dried) by hot carbon dioxide gas from conduit 36. Themoist carbon dioxide from the unit being regenerated is recycled intothe conduit after the first compressor 23 through conduit 37.

Feed gas, with a water content of approximately 50 ppm or less is fedvia conduit 38 to the final stage compressor 39 and cooler 40. The feedgas leaves compressor 39 at the maximum pressure of the process (39being the final compression stage) and is cooled by cooler 40 whichrejects heat to sea water.

The liquid CO₂ then passes via conduit 41 to the removal of volatilescolumn where the volatiles are removed by distillation. The volatilesare removed in the top of the column leaving the bulk of the CO₂ in theliquid phase. Liquid carbon dioxide is drawn off through conduit 43. Inorder to enhance the removal of volatiles a re-boiler 44 is attached atthe bottom of the column. The re-boiler provides heat in the bottom ofthe column to boil off volatiles, and thereby enhance the separation ofvolatiles from the CO₂. To enhance the recovery of CO₂ in the volatilerich gas stream at the top of the column a condenser is placed in thetop of the column. The required cooling duty for the condenser isprovided by vaporisation of liquid CO₂ at intermediate or productpressure.

The remaining liquid carbon dioxide passes through heat exchanger 45 toexpansion unit 46 which generates cold carbon dioxide gas and carbondioxide liquid. The liquid is directed via conduit 47 and heat exchanger48 into the final expansion tank 49 in which it is the desiredtemperature and pressure. The gas is split, part flowing via conduit 50back through heat exchanger 45 and thence via conduit 51 to heatexchanger 28 and part via conduit 52 through heat exchanger 53 andthence via conduits 54 and 51 to heat exchanger 28. Heat exchange 53serves as a condenser for column 42.

The gas formed in the final expansion tank 49 is fed via heat exchangers48, 28 and 55 to a heater 56 at which it is heated to a temperaturesufficient to regenerate the water absorption units 35 a and 35 b.

The liquid carbon dioxide in expansion tank 49 may be drawn off viaconduit 57 to a transport vessel.

In the embodiment shown in FIG. 1, the pressure and temperature beforeand after compressor 2 are preferably 5 bar A/25° C. and 11 bar A/25° C.The pressure and temperature in expansion tank 15 is preferably 6.5 barA/−50° C.

In the embodiment shown in FIG. 2, the pressures and temperatures at thesites marked A, B, C, D, etc. are preferably as set out in Table 1below:

TABLE 1 Flow location Pressure (bar A) Temperature (° C.) A 1.1 25 B 1.125 C 5 140 D 4.5 20 E 4.5 20 F 20 140 G 19.5 20 H 19.5 10 I 19.5 10 J19.5 10 K 60 180 L 60 20 M 60 18 N 60 −15 O 21 −20 P 21 −20 Q 21 −22 R6.5 −50 S 6.3 −27 T 6.1 −5 U 5.9 200 V 5.7 400 W 5.5 200 X 20.5 −22

The following three Examples refer to alternative ways in which theprocess can be operated with respect to heat rejection above or belowthe critical point of the feed gas.

EXAMPLE 1 Heat Rejection to Sea Water/Atmosphere below the CriticalPoint

The carbon dioxide is compressed from the supply pressure of 1 bar to amaximum pressure of approximately 60 bar in 3 compression stages.Between each compression stage the feed gas is cooled using sea water oratmospheric air. The fully pressurised feed gas, i.e. the output fromthe final compressor, is condensed with a heat exchanger again using seawater. The condensed feed gas is expanded to the transport pressureusing an expansion valve and communicated to the flash tank orseparator. In the separator the liquid phase is removed and forwarded toa transport or storage vessel and the gas phase is returned to thecompression stage.

EXAMPLE 2 Heat Rejection to an External Cooling Circuit below theCritical Point

The feed gas is compressed from the supply pressure of 1 bar to amaximum pressure of approximately 25 bar in 2 compression stages. Theintermediate cooling (between compression stages) is achieved using seawater or atmospheric air. The pressurised feed gas is then condensedusing a heat exchanger connected to an external cooling circuit. Thecondensed feed gas is then expanded using an expansion valve to thetransport pressure and communicated to a flash tank or separator. In theseparator the liquid phase is removed and forwarded to a transport orstorage vessel and the gas phase is returned to the compression stage.

EXAMPLE 3 Heat Rejection to Sea Water/Atmosphere above Critical Point

The feed gas is compressed from the supply pressure of 1 bar to amaximum pressure of approximately 85 bar (i.e. above the criticalpressure of 73.8 bar) in 4 compression stages. The intermediate cooling(between compression stages) is effected using sea water or atmosphericair. The pressurised feed gas is then cooled in the super-critical phaseusing sea water or atmospheric air. The pressurised fluid is thenexpanded from the supercritical phase into the two-phase region to thetransport pressure using an expansion means and communicated to a flashtank or separator. In the separator the liquid phase is removed andforwarded to a transport or storage vessel and the gas phase is returnedto the compression stage.

1. A process for the production from a feed gas which comprises carbondioxide of liquid carbon dioxide at a desired temperature and pressurewhich temperature is below ambient, above the triple point temperaturefor carbon dioxide and below the critical point temperature for carbondioxide and which pressure is above ambient, above the triple pointpressure for carbon dioxide and below the critical point pressure forcarbon dioxide, said process comprising: feeding said feed gas into theentry port of a liquefaction apparatus having a flow path from saidentry port to an exit port connected to an expansion chamber; flowingsaid gas as a fluid along the flow path through said apparatus andsubjecting said fluid to a plurality of compression and cooling cycleswhereby to generate liquid or super-critical carbon dioxide having atemperature and pressure above said desired temperature and pressure;passing said liquid or super-critical carbon dioxide through said exitport into said expansion chamber whereby to generate in said chambergaseous carbon dioxide and liquid carbon dioxide at said desiredtemperature and pressure; and recycling said gaseous carbon dioxide intofluid flowing through a said compression and cooling cycle; andoptionally withdrawing said liquid carbon dioxide at said desiredtemperature and pressure from said expansion chamber.
 2. A process asclaimed in claim 1, wherein one or more of the compression cyclesadditionally involves an expansion step.
 3. A process as claimed inclaim 1, wherein the fluid flowing to each compression cycle ismonophasic.
 4. A process as claimed in claim 1, wherein the expansionchamber is provided with a liquid removal port through which liquidcarbon dioxide is withdrawn.
 5. A process as claimed in claim 1, whereinthe recycled carbon dioxide passes through one or more heat exchangers.6. A process as claimed in claim 1, wherein the recycled carbon dioxideis returned to the fluid flow at an upstream point.
 7. A process asclaimed in claim 1, comprising 4 compression cycles.
 8. A process asclaimed in claim 1, wherein water is removed after at least onecompression cycle and before the final compression cycle.
 9. Apparatusfor carbon dioxide liquefaction comprising a flow channel for carbondioxide passage from an inlet port to an outlet port, said channelcomprising a plurality of compressors and coolers arranged in series,with an expansion chamber in said flow channel downstream of the finalcompressor and cooler and with a recirculation channel arranged toreturn gaseous carbon dioxide from said expansion chamber into said flowchannel upstream of said final compressor and cooler.
 10. Apparatus asclaimed in claim 9, wherein the expansion chamber is provided with aliquid removal port such that liquid carbon dioxide can be withdrawn.